Stable Ca isotopic compositions of global carbonatites and implications for the evolution of the Earth's upper mantle

Stable Ca isotopic composition (d^44/40Ca) of crustal carbonates as well as Ca bearing secondary crustal silicate sediments (e.g., clay) typically show lighter d^44/40Ca than the bulk silicate Earth (BSE) or the upper mantle composition (~0.94 ± 0.12‰). Hence, d^44/40Ca of mantle-derived rocks can be used as a potential tracer of recycled components into the mantle causing large-scale chemical heterogeneity. We report d^44/40Ca (w. r. t. NIST SRM915a) of relatively young (<200 Ma, n = 29) and older (>200 Ma, n = 16) carbonatites from different continents to investigate recycling of crustal sediments into the mantle and temporal evolution of Ca isotopic compositions of the upper mantle. The analytical uncertainty of the measurements of d^44/40Ca, performed using double-spike TIMS at CEaS, IISc, is better than 0.08‰ (2SD). The d^44/40Ca values of most of the older (> 200 Ma) carbonatites show a constricted range (0.79-0.9‰) overlapping with the BSE composition, whereas, the younger (< 200 Ma) carbonatites show much wider range with many samples showing lower-than-BSE d^44/40Ca values (0.44-1.12‰). This observation is also consistent with their ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic compositions, where, the younger carbonatites have much wider compositional variability than the older carbonatites. Based on the radiogenic isotopic compositions, it is suggested that the older carbonatites are primarily derived from a long-term depleted mantle source, whereas, there is a significant contribution of an enriched mantle component for the younger carbonatites. Such enrichment in the mantle source of the younger carbonatites is explained by recycling of crustal material. Our d^44/40Ca data also suggest that the older carbonatites are derived from depleted mantle source whereas, enrichment of mantle, due to more crustal input (having lower d^44/40Ca values), is prominent for the generation of younger carbonatites. This study suggests that the d^44/40Ca of the upper mantle has evolved through time.